Consideration as to producing sufficient homogeneous, hardenable low alloy powdered steel for processing as preforms for hot forming or as sintered shapes involves either or both of two procedures: pre-alloying or admixing. Pre-alloyed powders are currently in use as the basic material for low-alloy steel preforms or compacted shapes because of their homogeneity. However, pre-alloyed powders are relatively expensive compared to iron powder or conventionally produced iron and it is unlikely that parts producers will accept the limited number of alloyed compositions commercially available. Accordingly, pre-alloyed powders properly represent only one of several means of providing a full range of alloy preforms which are substitutional for conventionally made wrought alloy compositions. Mechanical mixtures of powders, hereafter referred to as admixtures, have been deemed capable of providing alloying during sintering of the precompact, but exactly how to achieve adequate homogenization of the allowing ingredients is not known to the prior art. The prior art recognizes that conceptually, admixtures seem to offer substantial economic advantages over pre-allowed powders. Complete flexibility should result from blending a base powder with a master alloy powder and thereby great reduction in manufacturing costs. To arrive at this goal, there must be optimization of the master alloy powder and the total admixture must be designed to improve the kinetics of the sintering process.
A variety of mechanism are at hand to produce the alloying condition by diffusion with degrees of success. For example, solid state particle diffusion can be used, diffusion resulting from gasification of one of the components to the admixture is feasible, or liquid phase sintering of the master alloy portion can be employed. Since diffusion in the solid state particle condition is limited by the number of the inner particle contacts, the hope of increasing the kinetics of complete alloying is limited. However, if the master alloy ingredient is converted to a gas or a liquid, there is an increase in the inner particle contact. Very few elements can be considered for the technique of gasification of one of the components and thus this avenue is relatively narrow in application. Therefore, there is a need for exploration and development of a master alloy powder which will function by the liquid phase method of sintering.
The use of an iron-carbon cutectic as a base for a master alloy to behave much as copper in a standard production alloy during sintering was known more than 20 years ago. Unlike nonferrous alloying additions, these master alloys were found to have greater solubility. However, certain problems that be overcome if the advantageous solubility of master alloys is to be utilized. The ingredients of such master alloy powder must be selected with care so that each of the ingredients is compatible one with the other, and the melting range of the master alloy powder must be relatively narrow and as low as possible; the master alloy powder must have good fluidity and wetting characteristics to facilitate cotaing of the base ferrous powder with the alloy liquid for purposes of facilitating rapid and effective sintering and diffusion through a minimum distance.